1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus furnished with an improved contact charger capable of producing higher-quality images.
2. Description of the Related Art
Conventionally, a corona discharge unit having a scorotron charger has been the leading electrophotographic charger.
Since corona charging utilizes the effect of electrical discharge, however, a large quantity of harmful ozone is produced in negative charging, in particular. Moreover, the applied voltage is relatively high, ranging from -4 to -5 kv, so that most of current flows through the casing of the unit, and a great energy loss is caused.
Recently, therefore, contact charging which hardly produces ozone has been developed as a technique to replace the corona charging. Typical contact charging systems include a roller charging system using an electrically-conductive roller and a brush charging system using an electrically-conductive brush. In either case, the quantity of ozone produced is believed to be one hundredth of the value for a corona charger, and the applied voltage is as low as about -1 kV. Thus, no current flows through the casing, so that the energy loss is small.
However, the roller charging system is susceptible to toner and dust, such as paper dust, which may directly cause unevenness of charging, thus affecting the resulting image. Moreover, this system has a complicated construction, requires high mechanical accuracy, and therefore, is expensive.
On the other hand, the brush charger is more resistant to soiling by toner and paper dust than the roller charger, and is low-priced. Accordingly, it is an effective charging means for use in a compact, inexpensive apparatus. Owing to its configuration, however, the brush charger is subject to a drawback that a number of white stripes are formed along the direction of movement of an object of charging when halftone printing is carried out in an electrophotographic process using a copying machine or printer, especially of the reverse developing system. In the negative-charging reverse developing system, the white stripes indicate a local elevation of the surface potential of the object of charging on the negative side. This phenomenon, which is attributable to unevenness of charging peculiar to the brush charger, occurs for a long period of time from the initial stage of use of the apparatus when a DC bias is applied in hot and humid ambient conditions. This phenomenon is marked when a brand-new brush is used, in particular.
Further, the production of white stripes peculiar to the brush charger can be reduced by superposing such a specific AC bias on the DC bias as uniform convergent charging of the object of charging is not caused (Japan Patent Application No. 5-66302). In this method, those portions with unduly increased potential are de-electrified in a reverse charging process using an AC bias. Thus, the halftone image quality, as well as the image quality in the initial state of the brush, can be made much better than in the case where only the DC bias is applied.
According to this method, however, the resistance of the charger to dirt is worsened. More specifically, toner and paper dust from a cleaning unit adhere to the charger, so that charge failure is caused and appears in the form of black stripes on the paper after preparation of thousands of prints.
In contact charging using a charging brush or the like, furthermore, the charging performance is often worsened by change of charging members with time. The surface potential changes by nearly 150 V in a predetermined period of time after the start of printing, so that the image density inevitably varies by a large margin during this period.